(1) Field of the Invention
The present invention relates to the manufacture of electrical and electronic circuits and particularly to the fabrication of printed circuits. More specifically, this invention is directed to conductive ink which may be employed to define paths for current flow between components of an electrical or electronic device. Accordingly, the general objects of the present invention are to provide novel and improved methods and materials of such character.
(2) Description of the Prior Art
Conductive inks are well known in the art. The typical conductive-silver-ink formulation contains a thermosetting binder which serves as a carrier for the silver particles. The binder may, for example, comprise an epoxy/urethane mixture. The conductive ink is applied to a nonconductive substrate by any suitable technique, to establish a current path or paths between components of an electrical or an electronic device, and the binder is thereafter cured by the application of heat.
The conductive inks disclosed in the prior art exhibit characteristics which make their use under certain conditions disadvantageous. Some devices, such as conductors, in which the conductive pathway is defined by a conductive ink, must be capable of withstanding "super soak" conditions, ninety-six hours at 95% relative humidity and 65.degree. Celsius, without degradation. Prior art conductive inks exhibit poor moisture durability. The bond between these inks and the various nonconductive substrates over which they are applied, to form a conductive pathway, is known to degrade under "super soak" conditions.
The prior art also discloses the use of conductive inks along with a circuit which has been etched from a copper to nonconductive substrate laminate. Here the laminate has undergone various wet processing manufacturing steps such as etching and stripping operations. Because of these manufacturing steps the now exposed cured adhesive which had been used to bond the copper to the substrate has been subjected to chemical attack. Application of prior art conductive ink has shown poor adhesion to the old chemically attacked adhesive.